The present invention relates to methods of obtaining high purity platinum from platinum alloys and, more particularly, to obtaining high purity platinum by electrolytic methods.
The present invention concerns an electrolytic process for extracting platinum of high purity from a concentrated hydrochloric acid solution of an alloy of platinum and Rh, Ir and/or Pd under simultaneous depletion of other noble and base metal impurities present in the alloy.
Platinum alloy is used in many industrial applications, for example in thermocouple elements, in catalysts for ammonia oxidation, in organic chemistry, in automobile exhaust catalytic converters, in dental technology and many other areas. Depending on the chemical and other production processes concerned, these alloys are sent to noble metal processing plants after a certain period of time in the form of scrap platinum alloy and are chemically separated and refined in those plants to obtain platinum.
The classical separation of platinum from Rh, Ir and/or Pd occurs by precipitation in the form of (NH.sub.4).sub.2 [PtCl.sub.6 ]. Due to the very similar chemical properties of the foregoing metals, however, this process involves a substantial amount of labor and is time-consuming.
The separation of the platinum from the iridium is particularly complicated, since both metals are present in the same stable valency (IV) and, during precipitation with NH.sub.4 Cl, form salts with almost identical properties.
A rough separation is only possible if the iridium IV cation is converted into the iridium III oxidation state. During subsequent precipitation of the platinum with NH.sub.4 Cl, coprecipitation of the iridium occurs nonetheless. Similar conditions are observable during separation of the platinum from the rhodium and palladium. The precipitated (NH.sub.4).sub.2 [PtCl.sub.6 ] contains large amounts of Rh and Pd. Reprecipitation or recrystallization steps are therefore required for further purification.
German Patent DE-PS 272 6558 describes a process for separating platinum from iridium by means of ion exchangers. This process merely results in platinum-containing iridium.
A large number of extraction processes are known for precipitation of platinum alloys, which, however, also require subsequent precipitation of the platinum metals.
All these processes require elaborate apparatus and technology and are therefore very cost-intensive.
Electrolytic processes for refining gold have been known for a long time (Gmelin Au, Syst. No. 62, 1949) and have been continuously developed (European Patent EP 0 253 783).
British Patent GB-PS 157 785 and the German Published Patent Application 594 408 describe electrolytic platinum refining processes, which partly use combinations of chemical and electrolytic process steps.
These processes are all very time-consuming and cannot be reproduced in a technically acceptable form in all aspects.
U.S. Pat. No. 4,382,845 describes a partial electrolytic separation of palladium from solutions containing an excess of palladium. Precipitation, however, is only possible up to the threshold at which platinum and palladium are present in equal quantities. The precipitation of remaining base and noble metals is not mentioned in this process.
In the known process for precipitating platinum and palladium the electrolysis cell comprises a cation exchanger membrane whose advantages are, however, not apparent, since platinum and palladium can also be precipitated without a cation exchanger membrane in the described concentration ratio and voltage range. Moreover, this process displays the same disadvantage as all other known processes, since it can only be operated with a maximum concentration of .ltoreq.100 g/l.